Studies carried out with human subjects have repeatedly demonstrated that genetic factors play a major role in regulating why some people smoke, and others do not. Other studies have demonstrated that sensitivity to the toxic actions of nicotine plays a major role in determining who smokes. Sensitivity to toxic actions also affects daily tobacco intake of smokers; smokers decrease tobacco intake when toxic responses develop. Previous studies funded by this grant have used the mouse to determine that genetically determined variations in nicotinic receptor genes regulate differences in initial sensitivity to nicotine. We have also shown that genetic factors influence the development of tolerance to nicotine following chronic treatment. The studies outlined in the first specific aim of this competing renewal application will use several nicotinic receptor subunit gene knockout (null mutant) mouse strains to identify the subunit compositions of native (naturally occurring) neuronal nicotinic receptors that are measured with several binding and functional assays. Studies outlined in the second specific aim will use these null mutant mice to determine the effects of chronic treatment with nicotine (multiple doses will be tested) on the number and functions of several nAChR subtypes. The relationships between chronic nicotine-induced changes in receptor numbers and function and altered behavioral responses to nicotine will be determined. These studies will provide a broader understanding of nicotinic receptor mediated mechanisms that modulate the development of tolerance, and perhaps sensitization, to behavioral responses produced by nicotine. [unreadable] [unreadable]